As a fuel assembly for fuel burn-up extension, there has been proposed the structure which is shown in FIGS. 1 and 7 of Japanese Patent Laid-Open No. 217186/1987. This fuel assembly is arranged at its central portion with two water rods of large diameter and therearound with a number of fuel rods. This fuel assembly has its seven fuel rods replaced by the two larger water rods. With this structure, the fuel economy can be improved because the number of fuel rods replaced by the water rods is small. Moreover, the fuel assembly can reduce a local power peaking.
Another structure for the fuel assembly of high burn-up exposure is disclosed in Japanese Patent Laid-Open No. 202386/1985. This fuel assembly has its fuel rods arrayed at a small pitch at its peripheral portion. The structure shown in FIG. 3 of Japanese Patent Laid-Open No. 202386/1985 will be described in the following. In this fuel assembly, two water rods are arranged at the central portion whereas fuel rods and water rods are arranged in the form of a square lattice so that the pitch of the fuel rods is large at the central portion where the utilization of thermal neutrons is low but is high at the peripheral portion where the utilization of thermal neutrons is high. The small pitch of the fuel rods in the peripheral portion improves the average utilization of thermal neutrons of the fuel assembly, thus leading to the high burn-up exposure of the fuel assembly.